Field of the Invention
The present invention relates to monitoring the operation and performance of sucker rod pumped wells. More particularly, the present invention relates to systems that employ dynamometers, acoustic level measuring devices, and pressure sensors in conjunction with a computer to monitor in real-time, record, and display a wide range of information about various operational parameters in oil and gas wells that employ a sucker rod pump.
Description of the Related Art
Most wells utilize a pumping system to extract oil, gas, and water from subterranean well boreholes. The pumping system typically comprises a surface mounted reciprocating drive unit coupled to a submerged pump by a long steel rod, referred to as a sucker-rod. The submerged pump consists of a chamber, plunger, and a pair of check valves arranged to draw fluids into the chamber and lift fluids to the surface on each upstroke of the plunger. Since wells range in depths to many thousand feet, the forces and pressures involved in the pumping operation are substantial. The costs of drilling, assembling, and servicing such wells are also substantial. Costs are only offset by efficient production of oil and gas products from the well. Thus, the careful attention given by operators to efficient and reliable operation of sucker-rod pumped wells over many decades of experience can be readily appreciated. Well operators can directly access and monitor surface mounted well equipment performance. One technique is the use of a dynamometer that determines the position and forces on the sucker-rod at the surface level. Wells employ a wellhead assembly to seal the well fluids within a surface plumbing system. The reciprocating rod enters the wellhead assembly through a sliding seal, which requires that the rod be terminated at the surface level by a polished portion, commonly referred to as a polished rod. The surface dynamometer output has traditionally been a dynagraph that provides a two dimensional plot of force versus position of the polished rod, generally referred to as a “surface card”. However, many of the critical pumping components are installed deep into the well's borehole, referred to as “down-hole”, where direct monitoring is not economically practical. Since failure of a system component down-hole can have catastrophic implications both in terms of repair costs and lost production, well operators have long sought equipment and techniques for assessing down-hole operation and performance. Experienced operators can, to a limited degree, extrapolate from trends in surface card plots over time to infer potential problems occurring down-hole, although this approach remains substantially subjective.
An important advancement in the area of down-hole performance analysis was contributed by S. G. Gibbs when he deduced that down-hole forces and movements could be accurately estimated based on structural information about the well equipment and surface forces and movement of the polished rod. Essentially, Gibbs modeled the sucker rod as a transmission line using a viscous damped wave equation in the form of boundary conditions to a set of differential equations. Gibbs' teachings were initially presented in U.S. Pat. No. 3,343,409 to Gibbs, issued Sep. 26, 1967, for METHOD OF DETERMINING SUCKER ROD PUMP PERFORMANCE, which was directed to a process for determining the down-hole performance of a pumping oil well by measuring data at the surface. The size, length and weight of the sucker rod string are determined and the load and displacement of the polished rod as a function of time are recorded. From that data it is possible to construct a load versus displacement curve for the sucker rod string at any selected depth in the well. Thus, Gibbs presents a technique for generating a pump level dynagraph, referred to as a “pump card”, according to surface measurements.
Further advancements in equipment and techniques for gathering and processing surface data and generating down-hole data have been contributed by McCoy et al., and are presented in a series of patents. The use of an accelerometer and strain gauge in a polished rod transducer to implement a surface dynamometer have been taught. The accelerometer advancements are presented in U.S. Pat. No. 5,406,482 to McCoy et al., issued Apr. 11, 1995, for METHOD AND APPARATUS FOR MEASURING PUMPING ROD POSITION AND OTHER ASPECTS OF A PUMPING SYSTEM BY USE OF AN ACCELEROMETER, which teaches that an accelerometer is mounted on the pumping system unit to move in conjunction with the polished rod. An output signal from the accelerometer is digitized and provided to a portable computer. The computer processes the digitized accelerometer signal to integrate it to first produce a velocity data set and second produce a position data set. Operations are carried out to process the signal and produce a position trace with stroke markers to indicate positions of the rod during its cyclical operation.
The McCoy et al. advancements in the use of a strain gauge in a surface dynamometer are presented in U.S. Pat. No. 5,464,058 to McCoy et al, issued Nov. 7, 1995, for METHOD OF USING A POLISHED ROD TRANSDUCER, which teaches that a transducer is attached to the polished rod to measure deformation, i.e., the change in diameter or circumference of the rod to determine change in rod loading. The transducer includes strain gauges, which produce output signals proportional to the change in the diameter or circumference of the rod, which occurs due to changes in load on the rod. The transducer may also include an accelerometer. The change in load on the polished rod over a pump cycle is used in conjunction with data produced by the accelerometer to calculate a down-hole pump card according to the teachings of in the prior art cited herein. The pump card showing change in pump load is adjusted to reflect absolute rod load by determining an appropriate offset. Various ways to determine the offset are available. Since the pump plunger load is zero on the down stroke when the upper check valve, called the traveling valve, is open, the value necessary to correct the calculated minimum pump value to a zero load condition may be used as the offset. The offset can also be estimated by either a calculation of the rod weight, a predetermined rod weight measurement or an estimated load value by the operator.
A typical well is produced by drilling a borehole and installing a well casing. A tubing string is lowered into the well casing, and the well fluids are pumped to the surface through the tubing string. Thus, there exists an annular space between the casing and the tubing. The well fluids are present in this space, and it is useful to know the liquid level of the well fluids to better understand well operations and to improve accuracy of certain measurements and calculations. In this regard, McCoy et al. have also provided further advancements in the art of measuring well casing and tubing liquid levels. These teachings are presented in U.S. Pat. No. 5,117,399 to McCoy et al., issued May 26, 1992, for DATA PROCESSING AND DISPLAY FOR ECHO SOUNDING DATA, which is directed to an echo sounding system with a acoustic gun which is mounted to the wellhead of a borehole casing. The acoustic gun produces an acoustic pulse that is transmitted down the casing or tubing. The acoustic pulse produces reflections when it strikes the tubing collars and the surface of the fluid. A microphone detects the reflections to produce a return signal. This signal is digitized and stored. The digitized signal is processed to detect the rate of the collar reflections, downhole markers and other structures in the well, and the stored signal is narrowband filtered with a pass band filter centered at the rate of receipt of the collars. The data signal is further processed to determine the time of occurrence of the acoustic pulse and the liquid surface reflection. Each cycle of the narrowband filtered signal corresponds to one collar reflection. In this signal, each cycle is counted, and extrapolation used when necessary to produce a collar count extending from the surface to the liquid surface. This is multiplied by the average joint length to produce the depth to the liquid surface.
As will be appreciated upon review of the aforementioned prior art and discussion, the process of monitoring well performance involves a visit to the well site by a technician for the operator, connection of the test sensors to the well's surface equipment, operation of the well, gathering data, reviewing prior production information, processing the data, and then analyzing the results. This is a highly technical process, and it requires a high degree of skill and knowledge to study a surface card, pump card, and liquid level data to develop a sense of the down-hole function and the overall performance of the well and pumping system. These efforts are essentially directed to determining how the well is performing in terms of the volume of well fluids produced in view of the pumping system's capabilities, and also determining if there are any performance irregularities developing that suggest a reliability issue or potential catastrophic failure event.
A discrepancy in the volume of fluids being produced is generally identified by a mismatch between the volume increase in the local storage tank over time, and the theoretical displacement of the pump based on pumping speed, stroke length and plunger diameter, and other physical and performance factors. There can be a number of reasons for such a mismatch. The petroleum formation reservoir may be providing insufficient liquid to fill the pump. Or, there may be a mechanical failure of the rod string, tubing string, a valve leakage, plunger slippage, inadequate design, improper pump system operation, and so forth. All of these factors can lead to a reduction in the volumetric efficiency of the pumping system. Further, since the well produces raw well fluids that contain oil, gas, water and solid minerals, there may be interference with expected operation of the pump. The pump inlet may be clogged, the valves may be partially blocked or restricted in movement. There may be excess gas in the pump chamber, creating a gas-locked condition.
With regards to the question of performance irregularities and potential failure of the pumping system, this issue is always present in the operator's mind and more prominently when the mean time between failures for a certain well has been less than expected. Although the tools available to the operators for analysis of dynamometer records have improved over the decades, as was mentioned in the foregoing discussion, these tools generally still focus on providing numerical results that the operator must interpret to obtain the desired information. Furthermore, this type of subjective analysis requires significant experience and is confusing and inaccurate as far as establishing how the pump is actually operating and the cause of unusual results. A major difficulty is created by the dynamic loads that are dependent on pumping speed and cause oscillations of the surface loads that are not directly caused by pump operation.
It is noteworthy that the availability of battery powered laptop computers outfitted with integrated circuits for analog to digital conversion and advanced analysis software has been instrumental in providing the benefits of digital dynamometer analysis technology to well operators. McCoy et al. working in conjunction with Echometer Co. in Wichita Falls, Tex., provide such a system, referred to as Total Well Management (“TWM”), which embodies much of the aforementioned prior art teachings. In the TWM system, acquired data consists of digitized load and acceleration samples measured at the polished rod during an extended period of time to ensure that the operation of the pump has stabilized. This data is expected to be representative of the normal operation of the pump. Processing of the surface data to generate the corresponding pump dynamometer cards is undertaken after acquisition of several strokes has been completed. The surface cards and pump cards can then be studied to analyze system performance. Additional structural and performance information may be presented together with the surface and pump dynagraphs.
FIG. 1 is a computer display output from the prior art Echometer TWM analysis software, and this figure shows the results of detailed calculations for a specific pump stroke that gives an analysis of the pump operation and the loads experienced at the surface. Note that the information presented in FIG. 1 is not produced in real-time at the time that the dynamometer measurements are taken. Rather, raw data is taken during the test, and then is later processed to generate to output of the display in FIG. 1. Pump displacement in this example is computed at 119.5 bbl/day based on the current pumping speed of 8.411 strokes per minute. The effective plunger stroke is 54.2 inches that corresponds to 62.65% of the total plunger stroke of 86.5 inches. Since the surface stroke is 100 inches there are 13.5 inches of stroke loss due to rod and tubing stretch. The shape of the pump dynagraph (bottom curve trace in FIG. 1) indicates that the pump barrel is filled with a mixture of liquid and gas at an initial pressure of 130.1 psi. The gas is compressed during the down stroke to a pressure that exceeds the pump discharge pressure at which point the traveling valve opens as indicated by the vertical dashed line. The minimum pump load is calculated as a negative 470 lbs, which shows that the bottom rods are loaded in compression. The polished rod power is computed as 6.3 HP from the area enclosed by the surface dynamometer card while the power expended at the pump equals 4.8 HP. The energy losses correspond to frictional forces between rods and fluids and rods and tubing. Additional analysis of the rod loading (not shown in this figure but presented in a detailed performance report) indicates that the rod string is loaded to 52% of the allowable loading, the pumping unit beam is loaded to 50% of its capacity and the gearbox is operating at 55% of maximum torque rating, and the prime mover is not overloaded.
Even though the prior art TWM system provides a substantial amount of technical information on well performance, it still requires a high degree of experience to interpret and analyze the numerical and graphical information in order to arrive at reasonable conclusions as to whether the pumping system is operating as intended and at the desired rate in an efficient manner. It also provides an after-the-fact analysis of a previously run test operation before the data presented in FIG. 1 can be presented to the user. Thus, is can be appreciated that there is a need in the art for a system and method for use in the sucker-rod pumped oil and gas well industry that further assists operators in calculating, analyzing, and outputting data while the sucker rod pump is in use, and providing a real-time representation of the facility function both at surface level and down hole.